In the field of wireless communication systems, power control is needed in a radio access network to allow the transceivers in a base station (referred to as a Node-B in a 3rd generation partnership project (3GPP™) communication standard within the universal mobile telecommunication system (UMTS™)) and the transceivers in a subscriber wireless communication unit (referred to as a user equipment (UE) in a 3rd generation partnership project communication standard) to adjust their transmitter output power level to take into account the geographical distance between them. The closer that the subscriber wireless communication unit (UE) is to the base station's (e.g. Node B's) transceiver, the less power the UE and the Node B's transceivers are required to transmit, for the transmitted signal to be adequately received by the other communication unit. Such a transmit ‘power control’ feature saves battery power in the UE and also helps to reduce the level of potential interference within the communication system. Initial power settings for the UE, along with other control information, are typically set by the information provided on a beacon physical channel in each particular communication cell.
Transmit power control systems that employ quadrature (I/Q) based power detectors, such as the transmit power control system 100 of FIG. 1A, can suffer from degraded accuracy due to a presence of quadrature imbalance in either the transmit (Tx) path and/or a feedback detector path. The transmit power control system 100 of FIG. 1A comprises a digital baseband integrated circuit (DBB) 105 comprising a root raised cosine (RRC) filter 110 designed to extract quadrature I/Q symbols from a wideband code division multiple access (WCDMA) signal input thereto. The extracted quadrature I/Q symbols are input to a reference quadrature balancing block 115, which provides reference quadrature balanced symbols 135 to a transmitter (TX) 120. The transmitter 120 is coupled to a gain estimation block 125 comprising a quadrature detector 130 and a reference path that also receives a representation of the pre-transmitter reference quadrature balanced symbols 135. The quadrature detector 130 is in, or coupled to, a feedback path that receives a portion of the transmit power amplifier output via, say, a directional coupler. The quadrature detector 130 and reference path both comprise anti-aliasing baseband low-pass filters 126, 127 to remove any out-of-band components (which may include all unwanted signals such as images, distortion products, noise, etc.) that may arise. The low pass filters 126, 127 provide filtered quadrature signals to an analogue multiplexer 140 that selects either: a reference I/Q signal, or a detected I/Q signal to pass through to an analogue-to-digital convertor (ADC) 145. The I/Q signals input to the ADC 145 are converted into digital form, and the digital form is filtered in adjacent channel interference (ACI) filter 150 to again remove unwanted components. In some known implementations, the analogue multiplexer 140 may be replaced with duplicate ADCs 145 and ACI filters 150.
The filtered digital representation is then input to a gain estimation algorithm 155, which calculates a gain value 160 to apply to the transmit amplifier gain chain. This gain estimate is used to correct the gain in the transmitter 120 in terms of the transmit power control (TPC) loop, for example to set the desired transmit output power. In essence, a TPC loop to accurately set an output power of the power amplifier may be considered as follows. The baseband or reference power level is typically known, as it is set by the design and varies with different uplink data rates within the 3rd generation partnership project (3GPP) of communication standards, more than one transport channel may be used, where a combination of Transport Formats (TFs) for all transport channels form a Transport Format Combination (TFC) that are identified in a Transport Format Combination Indicator (TFCI).
A typical value for the baseband or reference power level may be, for example, −6 dBm. Thus if we want the output power of the power amplifier to be, say, 24 dBm a gain of 24−−6=30 dB between the baseband and the antenna is required. If the gain between the baseband and the antenna is measured at, say, 29 dB, an extra 1 dB gain is required. Typically there will be a number of gain blocks along the transmit path that can be adjusted to realise the required gain adjustment. Ultimately, the accuracy of the TPC or power correction block reduces to the accuracy of this gain estimation. If there is an error in the gain estimation (or measurement), then this error will be transferred onto the accuracy of the output power, i.e. an xdB error in estimating the gain will translate into an xdB error of the output power. The gain estimation of gain estimation algorithm 155 is typically specified to have an accuracy of, say, better than +/−0.05 dB, whereas an accuracy of +/−0.2 dB for the calculated gain value 160 would be realistic.
An underlying problem that the present invention aims to solve relates to the fact that any quadrature imbalance along the reference transmit path and/or detector path can dominate the 0.05 dB gain estimation error budget, to such a degree that quadrature correction or balancing is required. The transmit quadrature imbalance may be corrected by the reference quadrature balancing block 115, such that any residual imbalance along the transmit path would result in the required image rejection. Therefore as far as the transmit signal and transmitter is concerned, image rejection is not an issue. However, the TPC reference signal 135 is tapped off after the TX quad correction step, and, hence, has phase and gain imbalance ‘inadvertently’ inserted into it by virtue of the transmit quadrature correction step, 115.
The use of a direct-conversion signal chain from the transmitter RF output down to a baseband signal in the feedback detector path provides a low-cost receiver solution for power control in third generation (3G) and fourth generation (4G) communication units. It is a less complex architecture than other receivers, not requiring the multiple surface-acoustic wave (SAW) and discrete filters used in a real intermediate frequency (IF) sampling architecture. The baseband channel filter in a direct conversion receiver is typically an integrated or discrete low-pass design that provides both out-of-band blocking and broadband noise rejection before digitization. It can be designed with much lower insertion loss and cost than the IF filters used in super-heterodyne or real IF sampling architectures. With an I/Q demodulator, the baseband cut-off frequency need only to be one-half of the total signal bandwidth for a complex modulated signal centred at 0 Hz. Despite these advantages, direct conversion radio design does not come without difficulty. For example, and in particular, any gain or phase imbalance on the ‘I’ and ‘Q’ paths, or producing a non-exact 90-deg. phase shift of the demodulator circuit, will result in energy at the unwanted sideband frequency.
Furthermore, in some power control systems, due to intentional transmit quadrature correction, an instantaneous amplitude and/or phase error is introduced to the power detector reference, as illustrated in graph 170 of FIG. 1B. Here, the ideal reference (including the intentional quadrature (I/Q) transmit path correction), is shown by line 174, and with reference correction of the transmit path quadrature imbalance 176 (undoing the transmit path quadrature error) added. The resultant signal comprises an instantaneous gain error (i.e. amplitude and phase) due to quadrature imbalance, as shown by line 172 (noting that any instantaneous phase error is irrelevant in this case as we are ultimately only concerned with (i.e. amplitude modulation (AM)) error gain. The severity of the quadrature imbalance problem depends upon the operational condition/statistics of the transmit data/channel. Over a finite (average) period of time, this instantaneous amplitude error leads to a degradation in the standard deviation accuracy of the detector, as illustrated by quadrature imbalance plot 186 in graph 180 of FIG. 1C, which shows a plot of standard deviation 182 versus detector power 184. The impact of the instantaneous amplitude error becomes more critical as the averaging time for the power estimation calculation is reduced. The problem may be further compounded due to a presence of any unknown transmit phase offset in the transmit path, as it is extremely complicated to correct for any such quadrature imbalance in both the transmit path and the feedback detector path.
It is known that the 3GPP™ standards impose very strict performance requirements on the TPC operation of communication units conforming to the 3GPP™ standards. In particular, the 3GPP™ standard specifies two forms of TPC, namely: open loop power control and inner loop power control. Open loop power control is an ability of the UE transmitter to set its output power to a specific value. It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network. In 3GPP™, the open loop power control tolerance is ±9 dB (normal conditions) or ±12 dB (extreme conditions). Inner loop power control (also called fast closed loop power control) in the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more TPC command(s) (TPC_cmd) that is/are received in the downlink, in order to keep the received uplink signal-to-interference ratio (SIR) at a given SIR target. The UE transmitter is capable of changing the output power with a step size of 1, 2 and 3 dB, in the slot immediately after the slot where the TPC_cmd can be derived.
As recent communication trends have necessarily been supporting ever-increasing broadband data rates, it is also known that certain high data rate channels are more sensitive to this problem, such that the accuracy of quadrature based power detectors is sometimes compromised in high speed data channels due to any quadrature imbalance.
US 2009/0196223 A1 discloses an example of a known transmit power controller that comprises a power detector that is based on quadrature detection, followed by a digital root mean square (rms) average calculation. This system is ideally suited to release 99 or voice channels of the third generation (3G) universal mobile telecommunication standard (UMTS™). However, the transmit power controller proposed in US 2009/0196223 A1 discloses an example that is inadequate for the higher data rate channels that are now supported in the high speed uplink packet access (HSUPA) of the 3rd generation partnership project (3GPP™) extension of UMTS™. In particular, US 2009/0196223 A1 fails to mention or disclose any mechanism to solve the problem arising from transmit calibration quadrature and detector imbalance. US 2009/0258640 A1 is a further example of a known transmit power controller that utilizes a quadrature based power detector and neither recognises nor offers a solution to the problem associated with quadrature imbalance.